struct default value

[drom]

Minimum minimorum:

module top ();

initial begin
	p1.lo = 4'h5;
end

endmodule

Failures:

tests/chapter-7/structures/unpacked/basic.sv
tests/chapter-7/structures/unpacked/default-value.sv
tests/chapter-7/unions/packed/basic.sv
tests/chapter-7/unions/basic.sv